1. Field of the Invention
This invention generally relates to a brushless DC motor structure with a constant ratio of magnetic rotor poles to the stator slots and particularly to one capable of reducing the cogging torque of the motor.
2. Description of the Prior Art
A rotor for a permanent magnet brushless motor is made up of multiple magnetic poles. Each arced magnet is fixed to a ferromagnetic steel ring and the radial direction of each arced magnet is opposite to that of the adjacent magnetic poles. The stator is formed by a plurality of ferromagnetic silicon steel sheets, and each ferromagnetic silicon steel sheet is provided with multiple winding slots for orderly wound copper wires (enameled wires). Due to the “slot effect” during motor running as the rotor rotates relatively to the stator, the cogging torque is generated to exert on the rotor. As the motor runs at a low speed, the effect of the cogging torque is extremely obvious and brings forth loud noise and vibration. As a result, the motor speed is limited to a minimum value to prevent the vibration.
U.S. Pat. No. 4,774,428 disclosed a three-phase permanent magnet motor that it has 3(2n+1) slots in the stator, 3(2n+1)+1 or 3(2n+1)−1 permanent magnetic poles in the rotor, and 2n+1 coils in the located slots for each phase. These designs enable to reduce the cogging torque and provide the maximum efficiency. The windings disclosed in the U.S. Pat. No. 4,774,428 are distributed 3(2n+1) slots to its stator and 2n+1 slots to each phase such that the 2n+1 successive groups of coils form one phase. As a result, each phase may have only one group of coils. For example, for n=7, the stator has 45 slots that corresponds to 46 or 44 poles of the rotor, and each one phase has 15 groups of coils. However, the resultant torque is not symmetric with respect to the shaft and bearing and will deteriorate the motor during operation, and this design is not considered as an ideal one.
U.S. Pat. No. 6,242,822 B1 indicated that the coils by interlaced winding for a star-connected stator are not wholly allotted to a magnetic period. That is, the ratio of the number of stator slots and the number of rotor pole pairs is a fraction, not an integer. (Two adjacent and opposite arced permanent magnets constitute one pole pair.) The aforementioned patent of the U.S. Pat. No. 4,774,428 in 1988 disclosed that the stator has 3(2n+1) slots in the stator, 3(2n+1)+1 or 3(2n+1)−1 permanent magnetic poles in the rotor. And its ratio of the number of stator slots and the number of rotor pole pairs is neither an integer. Therefore, the design of this non-integer ratio for the permanent magnet brushless motor is already a publicly known technique.